1. Field of the Invention
The present invention relates to a multiplexed audio-video transmission system through a pair-wire, such as used for connecting telephones, and using a main line branching device and to a branch line branching device enabling multiplex transmission of video signals and audio signals through the pair-wire, thereby enabling a point-to-multipoint connection model. This type of system is typically used in security systems using the pair-wire to provide two-way audio and one-way video viewing.
2. Description of the Prior Art
FIG. 10 shows a prior art audio-video transmission system such as applied in an audio-video security system.
The audio-video security system comprises a camera-equipped audio terminal 201 and a television-equipped audio terminal 217. The camera-equipped audio terminal 201, of which the exterior camera-equipped intercom unit, or interphone unit, is typical, transmits the multiplexed audio signal, DC current and video signal, which is modulated to a frequency band not overlapping the audio signal frequency band. The television-equipped audio terminal 217, of which the interior television-equipped intercom unit is typical, comprises an audio transmitter 215 for supplying DC current and receiving and transmitting the audio signal, and a video receiver 216 for receiving the video signal. The camera-equipped audio terminal 201 and television-equipped audio terminal 217 are connected by means of a specific wire pair 202a, 202b in the main line paired cable comprising plural wire pairs, a specific wire pair 207a, 207b in the branch line paired cable comprising plural wire pairs, the dedicated audio signal transmission cable 213a, 213b, and the dedicated video signal transmission cable 214a, 214b (in FIG. 10, only one pair each of the main and branch line paired cables is shown).
The main line wire pair 202a, 202b and the branch line wire pair 207a, 207b are connected by the main line branching device 205; the branch line wire pair 207a, 207b, audio signal transmission wires 213a, 213b, and video signal transmission wires 214a, 214b are connected by the branch line branching device 210. A main line terminator 204 is provided at the end of the main line wire pair 202a, 202b for the impedance matching of the main line wire pair 202a, 202b in the video signal frequency band and having a high impedance characteristic at frequencies lower than the video signal frequency band. A branch line terminator 208 is provided at the end of branch line wire pair 207a, 207b for the impedance matching of the branch line wire pair 207a, 207b in the video signal frequency band and having a high impedance characteristic at frequencies lower than the video signal frequency band.
The main line branching device 205 is connected at an appropriate position in the main line wire pair 202a, 202b and in turn to the branch line wire pair 207a, 207b. When the audio signal is passed to the branch line wire pair 207a, 207b, DC current is input from terminals 203a, 203b, the video signal is received balanced at a high input impedance from terminals 203a, 203b, and is output balanced to terminals 206a, 206b at an output impedance matched with the branch line wire pair 207a, 207b at the frequency band of the video signal.
The branch line branching device 210 is connected at an appropriate position in the branch line wire pair 207a, 207b, one end of the audio signal transmission wires 213a, 213b, and one end of the video signal transmission wires 214a, 214b. DC current is supplied from the terminals 211a and 211b. The audio signal is transmitted bidirectionally between terminals 209a, 209b and terminals 211a, 211b. The video signal is received at a high input impedance balanced from terminals 209a, 209b, and is output unbalanced to terminals 212a, 212b at an output impedance matched with the video signal transmission wires 214a, 214b at the frequency band of the video signal.
The main line branching device 205 and branch line branching device 210 of the prior art audio-video transmission system as above typically use a transformer.
FIG. 11 is a circuit diagram of a conventional transformer-type main line branching device. This transformer-type main line branching device 205 comprises a balanced transmitter 273 for transmitting the video signal, and a low band signal mixer 274 for transmitting the audio signal.
At the one balanced transmitter 273, the video signal input to terminals 203a and 203b passes capacitors 276a, 276b, is applied at a high input impedance with respect to the characteristic impedance of the specific pair-wire, to terminals 203a and 203b, supplied to the input-side coil of the transformer 277, of which the output impedance characteristic is matched to the characteristic impedance of the specific pair of wires used in the branch line cable, and is thus output from the output-side coil of the transformer 277 through capacitors 278a, 278b to terminals 206a and 206b. Note that passage of the audio signal and DC current is blocked at this time by capacitors 276a, 276b and capacitors 278a, 278b.
At the low band signal mixer 274, the audio signal is input through capacitors 280a, 280b and coils 281a, 281b, enabling bidirectional transmission between terminals 203a, 203b and terminals 206a and 206b. Coils 281a, 281b block passage of the video signal at this time, and capacitors 280a, 280b block passage of the DC current.
FIG. 12 is a circuit diagram of a prior art transformer-type door branching device 210. This transformer-type door branching device 210 comprises an unbalanced transmitter 284 for transmitting the video signal and balanced-to-unbalanced conversion, and a low band signal separator 283 for transmitting the audio signal.
At the unbalanced transmitter 284, the video signal input balanced to terminals 209a, 209b is applied to capacitors 289a, 289b and at a high input impedance to terminals 212a, 212b, passes transformer 290, of which the output impedance characteristic is matched to the characteristic impedance of the video signal transmission cable (of which coaxial cable is typical), and is output unbalanced from the transformer 290 to terminals 212a, 212b. The audio signal and DC current are blocked by capacitors 289a, 289b at this time.
At the low band signal separator 283, the audio signal is input through coils 286a, 286b and capacitors 287a, 287b to enable bidirectional transmission between terminals 209a, 209b and terminals 211a, 211b. The video signal is blocked by coils 286a, 286b, and the DC current is blocked by capacitors 287a, 287b at this time.
With the prior art transformer-type main line branching devices and branch line branching devices, however, it is necessary to use transformers 277 and 290 with a uniform high input impedance at the video signal frequency band, and manufacturing such transformers is technically difficult. This makes it necessary to use transformers with a nonuniform high input impedance in the video signal frequency band, resulting in deterioration of video signal transmission quality.
The transmitted signal level also tends to drop when the video signal is passed because the number of turns in the input-side coil of the transformer is significantly greater than the number of turns in the output-side coil.
In addition, the high cost of the transformers necessarily increases the cost of the overall system, while practical limits to reducing transformer size limit the size and weight reductions that can be achieved in the individual units.